With Minnesota teetering on the edge of battleground territory in the presidential race, Republican nominee Mitt Romney and President Obama sent in their top campaign surrogates Sunday, expanding the field in the closing days of a wildly unpredictable contest.

After largely sidestepping Minnesota other than a brief dinner stop last week, Romney running mate Paul Ryan attended a "victory rally" at Minneapolis-St. Paul International Airport, holding out the prospect of ending the state's 36-year stretch of voting for Democrats in presidential elections.

Hours later, former President Bill Clinton touched down in St. Cloud to energize DFL volunteers and shore up a firewall of Midwestern states, including Ohio and Wisconsin, which could play a critical role in picking Tuesday's winner. It was his second trip to Minnesota in less than a week.

Furthering the impression of a swing-state battleground, the Romney campaign announced that one of the candidate's sons, Josh Romney, will attend an election-closing GOP rally in Plymouth Monday morning.

"I've got a question, Minnesota," said Ryan, a Wisconsin congressman, firing up an enthusiastic crowd of 10,000 at a Sun Country Airlines hangar. "Are you going to help us win this election? ... We can use your help."

The last-ditch push of presidential campaigning added to a daylong mix of debates in Minnesota, including a face-off between U.S. Sen. Amy Klobuchar and her Republican opponent, state Rep. Kurt Bills, before a live audience on public radio.

Clinton's visit to St. Cloud -- following trips to Minneapolis and Duluth last week -- also aimed at the heart of the congressional district represented by GOP firebrand Michele Bachmann, who is locked in a surprisingly competitive congressional race with DFL businessman Jim Graves.

While the two presidential advance teams set up their events in Minnesota, Bachmann and Graves engaged in a spirited verbal duel on KSTP-TV, the last of three encounters in less than a week.

Graves, who later introduced Clinton at the DFL rally, called Bachmann "the most partisan, most polarizing person in Congress."

Bachmann, running ads calling herself an "independent voice," used Clinton's visit as an opportunity to add to the record $23 million haul she has taken in for her race. She told supporters in an "urgent" e-mail that "BILL CLINTON IS COMING TO MY DISTRICT TODAY TO PERSONALLY CAMPAIGN AGAINST ME."

Obama aides downplayed any connection between Clinton's trip and Bachmann, who has been a lightning rod of criticism for Democrats. "He's here to fire up the base as well as to thank the huge network of grass-roots supporters," said Obama spokeswoman Kristin Sosanie.

Graves acknowledged that it was more a personal favor than a marker in the presidential election. "We're friends," said Graves, who raised money for Hillary Clinton's 2008 presidential campaign. When Bill Clinton came to Minneapolis last week, Graves wasn't able to attend, so Clinton offered to come up to the Sixth Congressional District. Added Graves: "I don't think they would have sent him to St. Cloud, Minnesota, if they didn't think we could win."

"I'm here to support President Obama," Clinton told the shivering supporters who couldn't fit in the auditorium at St. Cloud State University. "But I'm also here to support what I hope will be our next congressman, Jim Graves."

For many Democrats, a chance to see Clinton also was a chance to recapture the enthusiasm of past presidential elections.

"I think it helps," said Dan Maranell of Waite Park, who got in line almost two hours before the doors opened. "Trust and respect. Those are the easiest things to lose and the hardest things to get back. This will help, to hear from somebody who did such a great job as president."

'This is our chance'

Bachmann, who fired up the Ryan rally at the airport, cited columnist George Will's prediction that Minnesota could shock the nation on Election Day. "Minnesota could prove to be a pivotal state," she said. "This is our chance."

Another speaker at the Ryan rally was Marvin Windows President Susan Marvin. She endorsed Romney as a "job creator," despite Obama's call-out to her company at the Democratic National Convention for not laying off workers during the recession.

The Obama campaign has given mixed signals about its prospects in Minnesota, a state where Romney has mounted no field organization and where the president has maintained a consistent if tightening lead in most polls.

While Obama's state campaign director, Jeff Blodgett, has long said he is taking nothing for granted, national campaign manager David Axelrod said last week that he will shave his signature mustache if Obama loses Minnesota.

The Romney campaign and its allies, which recently started airing television and radio ads in Minnesota, have been happy to portray the state as part of an ever-expanding contest that is tightening even in traditionally Democratic strongholds such as Pennsylvania and Wisconsin.

Minnesota Republican Party Chairman Pat Shortridge called Clinton's visit Sunday "a last-ditch effort to save a state they mistakenly took for granted. ... It's looking more and more likely that David Axelrod will be shaving his mustache on November 7."

Marlin Levison, Dml - Star Tribune

Former President Bill Clinton highlighted a political rally at St. Cloud State in St. Cloud on Sunday

Hope or Hail Mary?

For their part, Democrats were eager to portray Ryan's visit to Minnesota as a Hail Mary pass to salvage an election where Romney has been consistently behind, however narrowly, in a number of the battleground states.

"The Romney campaign has found itself with a tremendously narrow and improbable path to 270 electoral votes, so they are desperately trying to find a path through states like Minnesota," Sosanie said. "But the problem is, it doesn't exist."

The Obama campaign has been matching the GOP ads in Minnesota, although some analysts say both campaigns' ads are just as likely targeted at neighboring Wisconsin, which Ryan helped put into play.

Ryan's airport rally marked the Romney campaign's first major public event in Minnesota, though both have been here for private fundraisers. Ryan, accompanied by his family, was introduced by U.S. Rep. Chip Cravaack, a freshman Republican who is fighting to hold his northern Minnesota seat against DFL challenger Rick Nolan.

Among Romney's fans in the cavernous airplane hangar was Minneapolis construction salesman Matt Drew. "I don't think they'd spend the time and the resources here if they didn't have a chance," he said.

While the Ryan and Clinton events attracted state political luminaries in both parties, Klobuchar and Bills prepared for their last debate at St. Paul's Fitzgerald Theater. For Bills, struggling for attention against a popular incumbent, it was an opportunity to make Minnesotans notice a low-budget campaign that has barely blipped on the political radar. Public polls all year have given Klobuchar a dramatic edge in the race, while Bills, who rode a wave of Ron Paul supporters to the GOP nomination, remains a virtual unknown.

On many issues they staked out starkly different positions and styles for voters to choose. Bills said he didn't believe in climate change; Klobuchar said she does. Klobuchar highlighted her bipartisan work on the budget, the debt and veterans issues; Bills stressed the need for a newcomer in Washington to create change.

Staff writers Jennifer Brooks and Rachel E. Stassen-Berger contributed to this report. Kevin Diaz is a correspondent in the Star Tribune Washington Bureau.

Tending to party politics, President-elect Donald Trump is rallying supporters in Louisiana on Friday ahead of the state's Senate runoff election, aiming to pad the Republican majority he will inherit.